Here are the rules for the last competition:
That's the last one. There isn't a current one. I'm not sure if there will be one this year due to moderators dropping out. Plus you can go look through Polycount, CGTalk, 3Dtotal, etc to find the old threads which will also list the rules. You join by going to a site that's participating, and start a thread in the appropriate area. The main DW site will have a list of the qualified forums.
In order for ca.org to join, we'd need volunteer moderators and someone would have to contact the people running DW.
"Astronomy offers an aesthetic indulgence not duplicated in any other field. This is not an academic or hypothetical attraction and should require no apologies, for the beauty to be found in the skies has been universally appreciated for unrecorded centuries."